In the drilling and production of oil and gas wells it is frequently necessary to penetrate one or more production zones to reach an underlying zone with production tubing. Each production zone is served by its own string of production tubing which then must penetrate the overlying production zones to reach the surface. At the point where each string of production tubing penetrates an overlying production zone, the tubing can be subjected to severe erosion.
Particularly where the production zone being penetrated produces high pressure gas, abrasive materials entrained in the gas can quickly erode the surface of the production tubing penetrating that zone. High pressure gas moves at very high velocities when the zone is producing and it can contain such entrained erosive materials as grains of sand or drops of liquid. Where the high velocity gas enters the well bore, it impinges upon the penetrating production tubing from an underlying zone and, depending upon the material from which the production tubing is made, the high velocity gas can quickly damage or even penetrate the wall of the production tubing.
When the wall of the production tubing has been penetrated in an overlying zone, communication is established between two different production zones through the failed production tubing string. Communication between zones can be highly undesirable, and isolation between the zones must be maintained. Isolation between the zones in the well bore is generally obtained by the use of packers about the production tubing between production zones. When erosion of production tubing has reached an advanced state it becomes necessary to replace the eroded section or sections of tubing. This requires removing the production tubing string from the well bore, replacing the failed joints of tubing and reinserting the tubing string into the well bore along with any necessary replacement of packers.
Different tubing materials will erode at different rates and to different degrees depending upon the velocity of the impinging fluid as well as the type and amount of abrasive materials entrained in the fluid. Some materials are considered highly erosion resistant, and various methods have been used to incorporate these materials into the design of the production tubing where the tubing must penetrate overlying producing zones. Joints of pipe or tubing have been designed which incorporate a jacket of a highly erosion resistant material, such as tungsten carbide, over the production tubing. Various methods are used to insure that the tungsten carbide will surround the tubing at the areas where the high velocity fluid enters the well bore. The tungsten carbide is generally installed in stacks of relatively short rings and held in place longitudinally by various types of sleeves and collars. In some designs the rings are kept pressed together by the installation of a spring, such as a coil spring, around the tubing to press against the end of one of the tungsten carbide rings.
A major problem with the use of a highly erosion resistant material, such as tungsten carbide, is that such materials, while being very hard, are also very brittle and therefore subject to damage when placed under radial or axial loads. When making up a tubing string for insertion into a well, it is necessary to support the top joint on the string at the well head and to hold that joint against rotation while threading the next joint to the top joint of the string. Power tongs or similar tools are normally used to thread the two joints together. The top joint of the tubing string is held against rotation while the power tongs rotate the next joint being installed. Such power equipment necessarily bites into the surface of the tubing joint in order to apply the necessary torque to either hold the joint against rotation or to rotate the joint in making up the thread. If such power tools are used on a surface of the blast joint where the erosion resistant material, such as tungsten carbide, is exposed, the torque is not efficiently transferred to the tubing and the tungsten carbide rings can easily become chipped and cracked, resulting in a loss of protection against abrasion once placed into service in a production zone.
Another operation frequently encountered which can result in damage to the tungsten carbide rings is the gripping of a blast joint from above with pipe slips having upwardly canted internal teeth. These teeth must necessarily dig into the surface of the joint to achieve their gripping action in order to pull the blast joint or other piece of equipment out of the well bore. Gripping of a blast joint by such a slip, if the slip teeth contact the tungsten carbide rings, can result in cracking or breaking of the rings as mentioned before in the case of power tongs, by the application of radial or axial forces, through the slip teeth.
Some blast joints, using tungsten carbide rings, incorporate thin metal sleeves on the outside of the tungsten carbide rings, but these sleeves are of insufficient strength to withstand the radial and axial stresses imparted by power tools, and the sleeves typically are not mounted to the inner production tubing with sufficient mechanical strength to transfer the necessary torque or axial force to the tubing itself. These thin sleeves are generally only effective at protecting the tungsten carbide rings during assembly and handling which does not involve the use of the aforementioned power tools.
Another disadvantage of blast joints having exterior carbide rings occurs when the operator wishes to insert the blast joint through a snubber unit. A snubber unit allows a pressurized well to be reworked without first plugging or killing the well. Killing the well is undesirable because it can be difficult and expensive to resume production from a well that has been killed. The snubber unit is mounted on the wellhead of a well which is to be reworked under pressure. The snubber unit establishes and maintains a pressure seal around the tubular goods coming out of or going into the well at the wellhead. It is typically mounted atop one or more blowout preventers. In addition to maintaining the pressure seal, the snubber unit grips any tubular goods being inserted and forces them into the well against the wellhead pressure, which can approach several thousand pounds per square inch. Alternatively, the snubber unit can be used to grip a tubular good being extracted from the well to limit or control its outward movement under wellhead pressure.
This gripping of the tubular goods can be accomplished by pipe slips or other devices which place highly concentrated mechanical loads on the goods being gripped. In addition, the pressure seal can only be effective if applied to a relatively smooth surface which is capable of maintaining its pressure integrity under the pressures experienced at the wellhead. It can be seen then, that a typical carbide blast joint can not be inserted into a well with a snubber unit. If this were attempted, the carbide rings would immediately deteriorate or even fail completely, and the pressure seal could not be maintained. A very expensive and dangerous blowout would occur. For this reason, whenever it has been necessary to insert a blast joint during rework of a well, the well has been killed, rather than using a snubber unit.
Presently known blast joints, therefore, in addition to being very expensive, are somewhat fragile, and they cannot be installed in a string of production tubing as if they were simply another joint of production tubing. It is desirable to design a blast joint which provides protection against erosion for its full length without any gaps, but which can be handled by the use of power tongs and pipe slips without any unusual degradation of the joint. It is further desirable to design such a blast joint which can be threaded into place in a production tubing string as if it were simply another joint of production tubing, either as a single joint or as a string of consecutive joints having a continuous protective sheath of tungsten carbide rings without any gaps in between. Additionally, it is further desirable to design such a blast joint which can be used with a snubber unit.